1. Field of the Invention
The invention relates to a process for continuously synthesizing alkoxypolysiloxanes having the general formula ##STR1## in which R.sup.1 is an alkyl or aryl radical,
R.sup.2 is a lower alkyl radical with 1 to 4 carbon atoms,
a is not greater than 1.5, PA1 b is not greater than 2, and PA1 a+b is not greater than 3,
by solvolysis of chlorosilanes of the general formula EQU R.sub.a.sup.1 SiCl.sub.4-a
with, relative to the silicon-linked chlorine, an excess of alcohol and less than an equivalent amount of water, and partial condensation of the solvolysis products. The invention furthermore relates to equipment for carrying out this process.
2. Description of the Prior Art
Because of the alkoxy groups linked to silicon, alkoxypolysiloxanes are reactive compounds, which can be used either directly as such, for example, for impregnating purposes, or as reactive intermediates. Alkoxypolysiloxanes of the aforementioned formula I moreover represent valuable intermediates for curable silicone resins.
It is particularly important that such alkoxysiloxanes be produced in as uniform and reproducible a composition as possible by inexpensive means and by a process that can be carried out as simply as possible. Fulfilling these requirements is difficult because of the different reactivities of the silanes which depend on the functionality and substitution. A particular problem is that some trifunctional silanes tend to gel when reacting with water, even when the reaction is carried out in the presence of an excess of alcohol. Consequently, the gelled silanes are unavailable for the condensation reaction and since their availability cannot be controlled, the production of uniform alkoxypolysiloxanes becomes difficult. It is therefore particularly difficult to synthesize reproducible alkoxypolysiloxanes whose a-value is low, especially not greater than 1.5, since silane mixtures with a high proportion of trifunctional silanes or, for particularly low a-values, trifunctional silanes by themselves are used to synthesize such alkoxypolysiloxanes.
It is particularly desirable to synthesize alkoxypolysiloxanes with a low a-value in a continuous process, since the equipment expense for high space/time yields can be justified only with a continuous process.
Processes are known for synthesizing alkoxypolysiloxanes continuously. In these processes, it is attempted to counter the aforementioned difficulties due to high reactivity, especially that of trifunctional halogensilanes with water, by suppressing the reaction through the addition of an inert solvent and/or first converting the halogensilanes completely or predominantly into alkoxysilanes. The alcoholysis products which are formed at first, are reacted with less than equivalent amounts of water and partially condensed. The alcoholysis process is therefore separated from the hydrolysis process and from the condensation process and is carried out either in different zones of one apparatus or in separate reaction vessels. Moreover, the alcoholysis is usually carried out at elevated temperatures, i.e. at the boiling points of the alcohols used. In this reaction and under these reaction conditions, the predominant amount of the hydrogen halide formed is liberated and removed from the system.
The closest state of the art is represented especially by the following publications.
German Offenlegungsschrift 20 61 189 relates to a process for continuously synthesizing optionally substituted alkoxysilanes or optionally substituted alkoxypolysiloxanes by reacting chlorosilanes with optionally substituted alcohols and, optionally, with water in a column equipped with reflux condenser and kept at an elevated temperature. The process is characterized by the fact that the chlorosilane is introduced at the head of the column, the optionally substituted alcohol is introduced in gaseous form from below, or at a place between the lower end and the upper end of the lowest third of the length of the column, and the water, if used, is introduced at any place in the column. The reaction product is taken from the lower end of the column or below the place where the alcohol is introduced. The temperature of the column is maintained over the whole of its internal cross section and at least over two thirds of the distance between the place of entry of the alcohol and the place of entry of the silane into the column, at least 1/220 C. above the boiling point of the alcohol used at the pressure existing in the column. Excess alcohol, boiling under reflux, is constantly present at the head of the column during the reaction. As is evident particularly from the examples, the emphasis in this process is on the continuous synthesis of alkoxysilanes. For example, the methyltrichlorosilane, which is at the head of the column at room temperature, is fed into the column, which is heated to 100.degree. C. Below the place of introduction of the silane, ethanol is introduced in gaseous form at a temperature of 110.degree. C. The reaction of the ethanol with the methyltrichlorosilane therefore takes place at high temperatures. The alkoxysilane formed flows from the lower end of the column and is purified by distilling off the excess ethanol.
If partial hydrolysis of the methyltrichlorosilane is to take place at the same time, the methyltrichlorosilane, according to the example, is used in the form of its solution in toluene and the water is introduced in gaseous form at a temperature of 105.degree. C. into the column below the place where the silane is introduced. This process, however, does not produce a uniform reaction product of relatively high molecular weight, but a mixture of low molecular weight methylethoxy polysiloxanes and methyltriethoxysilane.
A further development of this process is described in German Offenlegungsschrift 28 06 036. The characteristic features of this process consist in that alcohol, water and, if necessary, an acidic catalyst which promotes the condensation of silicon-linked condensable groups, are constantly introduced into a still, which is equipped with a column and, if necessary, a reflux condenser, and which already contains the desired polysiloxane. The amounts added are such that there are at least 5 weight percent of the alcohol and 0.001 to 5 weight percent of the catalyst, these percentages being based on the total weight of the alcohol, catalyst and polysiloxane in the still. The contents of the still are heated so as to boil under reflux. Halogensilane, to be reacted is introduced into the column at least 1 m above the lower end of the column and polysiloxane is constantly drawn off from the still in proportion to its formation.
This process also uses a column with reflux condenser and still, which already contains the desired polysiloxane and to which alcohol, water and, if necessary, a catalyst, are constantly added in specific amounts so that when the process is running under conditions of reflux, the alcohol used for the alcoholysis, is boiling and the halogensilane is fed into the column at a fixed minimum distance from the lower end of the column. The alcoholysis therefore takes place in the column, while the hydrolysis and condensation take place after the alcoholysis in the still. Separation of the alcoholysis reaction, on the one hand, and of the hydrolysis/condensation reaction, on the other, is achieved with this process.
The process requires a relatively high expenditure of energy since it is necessary to work permanently under conditions of reflux. The throughput of the equipment is limited by the need to carry out the preliminary reaction in the upper region of the column.
A further modification of this process is described in European Patent Application No. 0 032 376. This process is characterized by the fact that chlorosilane and an aliphatic compound with an alcoholic hydroxyl group, each in liquid form, are introduced in amounts of 0.5 to 0.9 gram moles of alcoholic hydroxyl group per gram atom of silicon-linked chlorine in co-current flow in a first step into a first reactor, in which they are mixed together. The liquid reaction mixture emerging from the first reactor is introduced, in a second step, at the head of a column used as second reactor and maintained at an elevated temperature. An aliphatic compound with an alcoholic hydroxyl group is introduced in gaseous form in an amount at least sufficient to completely convert the remaining silicon-linked chlorine in the reaction mixture obtained in the first step, at the lower end of the column or at a place between the lower end and the upper end of the lower third of the column. Water, if necessary, is introduced at any place in the column and the reaction product is taken from the column at its lower end or from a place below where the aliphatic compound with an alcoholic hydroxyl group is introduced.
With this process, therefore, alcoholysis with a partial amount of the alcohol takes place in a first reactor. The remaining amount of alcohol is added to the product of the process in a second reactor, water being added to the second reactor, if condensation of the silanes to siloxanes is desired. This second reactor is once again constructed as a column, and the addition of the remaining amount of alcohol and, if necessary, of the water takes place in the heated column. The examples exclusively describe the preparation of alkoxysilanes. This process also has the disadvantages of requiring relatively expensive equipment and of having to maintain elevated temperatures, namely, at least the boiling point of the aliphatic alcohol used.
U.S. Pat. No. 3,688,180 describes a process for the preparation of alkoxyorgano polysiloxanes in which an organohalogensilane is first reacted in the presence of a hydrocarbon solvent, water and a lower aliphatic alcohol in a reactor at -10 to +10.degree. C. and the intermediate produced is supplied continuously to a second reactor, which contains additional solvent, the reaction being brought to conclusion there under reflux conditions. Here also, the disadvantages of the process lie in the need to use two reactors and, additionally, in the need to use solvents.